Why I'm always directed to the same bank when I make transactions via API?

The problem is caused by incorrect integration on the website.

Transactions are generate by the API "Create" method.

This method requires you to enter the bank group number choosen by customer, so you should provide the bank group selection on your website, before generating such a transaction. In order to fix integration, we recommend using our PHP libraries https://github.com/tpay-com/tpay-php (required version PHP 5.6 or higher)

Link to example where downloading a channel view is one command: https://github.com/tpay-com/tpay-php/blob/master/tpayLibs/examples/BankSelectionAPI.php